Parylene is very different to typical liquid conformal coatings like acrylic, epoxy, silicone and polyurethane resin based materials.

When you buy Parylene it is a white powdered dimer.

This powder cannot be applied to circuit boards in its current state. The dimer requires specialist Parylene equipment that uses a Chemical Vapour Deposition (CVD) process. That is the dimer is placed inside the machine, a vacuum created and the dimer is then transformed to a gas vapour by being heated up.

This vapour created during the pyrolysis process can then be deposited on the printed circuit board as the Parylene coating.

This is very different to liquid conformal coatings. They start off as a “varnish” in a can or container and are applied to circuit boards by brushing, dipping or spraying. They then dry either naturally or artificially in some manner and the circuit board is now protected.

Where to buy Parylene

When purchasing Parylene it is important to find a source that provides a quality product. The purity of the coating is one of the key points in creating a successful Parylene process. Neglecting this fact can cause endless problems.

Remember that buying Parylene cheaply is easy. Buying cheap Parylene that is pure enough to protect the circuit board is another issue.

Thin film Partners can inform you of the right vendors who supply high-quality material and the pricing for each type. Then you can compare which vendor you would like to buy from.

This is a cost effective method to ensure you get the right Parylene coating at the right price.

Want to find out more about Parylene?

Contact us to discuss your needs and let us explain how we can help you.

There are four key reasons why Parylene masking and de-masking is more difficult compared to liquid conformal coatings.

These are:

Parylene is a vapour. When you are masking against a gas rather than a liquid then there is more of a challenge. So you need to provide a much better barrier with the masking process compared to the liquid coatings.

Parylene is immersion. Most liquid conformal coatings are sprayed and so the capillary is less compared to immersion in a limitless supply of material.

Stripping Parylene is hard. It is much harder to remove unwanted Parylene material on components that should not have been coated. Parylene is chemically inert (therefore harder to strip off or remove) and more difficult to see (no UV trace in most Parylene coatings). Mistakes can be more costly.

The Parylene can bond more to the masking materials. When the Parylene is deposited on the masking materials and circuit board it can bind the two together and it can take significant effort and care to remove the masking materials without damaging the board or the Parylene coating integrity.

Using polyurethane conformal coatings to protect against chemical attack

But, what is key to their selection as a protective conformal coating is that they also offer excellent chemical resistance to the circuit board.

The reason for this is that UR coatings cure rather than dry. That is they cross-link once applied to the circuit board by one of the many methods including solvent evaporation (initial stages), heat, UV, moisture and catalysed cure.

This cross-linking of the polymer chain provides the chemical resistance for which the UR coatings are generally selected.

This makes them excellent conformal coatings where the chemical attack is a potential hazard for the electronic circuit boards. This includes sectors such as aerospace, military and industrial sectors plus other diverse areas such as medical and commercial electronics.

Again, engineers need to consider their options on whether to use conformal coating masking as soon as possible.

Pre-Coating Inspection

Generally, it is more efficient to double check the masking process is correct before conformal coating application rather than repair the PCB after the process goes wrong because the masking process was incorrect.

This check can be manual or automated but it is highly valuable.

Coating Application

The one stage that cannot be avoided is the conformal coating applied to the PCB.

Inspection

The conformal coating inspection process can be done manually or automatically. This can depend on the volume of PCBs and the level of sophistication required.

Again, it is down to operator training and using the right equipment to ensure that this is possible.

Also, at this stage it is possible to measure process factors like coating conformal thickness to check that the criteria are met.

Other factors to consider

The set up of a conformal coating production line regardless of the application method has many similar characteristics.

General requirements

Any coating facility will need the basic requirements put in place that would be standard for any piece of electronic manufacturing process. These include ESD systems, facilities for the machines, the environmental requirements and the normal Health & Safety (HSE) considerations.

Health & Safety (HSE)

HSE tends to be more important for conformal coatings since in general the coatings themselves are hazardous, or the way they are applied makes them potentially harmful to operators

Environment

Conformal coatings are sensitive to the environment that they are processed. Cleanliness could be critical, as can temperature and humidity.

Summary

Setting up a conformal coating production facility can be a straightforward process as long as all factors are considered.

Get this right and many of the problems that could occur during production will be avoided.

Nexus has been examining Molecular Vapor Deposition (MVD), a new novel coating technique that may be able to offer superior protection to electronic circuit boards compared to the standard coating technologies like conformal coatings and Parylene.